Refactor expression creation splitting indices and JuMP expressions

[abelsiqueira]

Description

Like variables and constraints, it might be possible to move the creation of (at least some of) the expression indices tables.
The multi-year expression indices, in particular, seem easy to move. The "intersection"/balance expressions will be more challenging.

[clizbe]

What's the benefit of this? :)

[abelsiqueira]

Separation of concerns, like for variables and constraints. The indices being created before means that no JuMP is involved, so it improves modularity of the code.